A novel approach has been used to produce monoclonal antibodies to tumor-associated antigens which may be of value for immunodiagnosis or prognosis of human tumors. We have established that human melanoma cells synthesize proteins that are serologically homologous but not molecularly identical to normal serum constituents. We have detected several such proteins in spent culture medium of human melanoma cells but have initially concentrated on Alpha2-macroglobulin, a wide-spectrum protease inhibitor. We have demonstrated synthesis, characterized the molecular form, and produced monoclonal antibodies to this tumor cell product. We are presently using immunization methods with insolubilized immune complexes to produce further monoclonal antibodies which may recognize molecular differences between the melanoma and serum forms of the Alpha2-macroglobulin. This methodology should produce a new category of monoclonal reagents which may not only be clinically useful but also may help to elucidate the functional roles of these homologous serum proteins.